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I don't know if this is supposed to happen in the game, but if you find a tree, rock, and those cacti-like plants in caves with a research pod on it, you can then take it off and put any one slot or two slot objects on there. I have tested this with resources and a medium storage.

Try to think this idea not as a seasoned Astroneer player, but as a green out-of-your-first-launch player.
As it is right now, the small rover is a beginner's trap. Why is it a trap? Well, first, what are the good points of the small rover? It is cheap to research, to build and is easy on fuel. What are you lacking at the beginning of a game and more so as a beginner? Research points, materials (particularly rares one), and energy sources. It is perfect for your first rover and an obvious choice for a beginner (it also has the "small" name on it ?).
The traps comes on the actual functionality of the small rover. It cannot carry research pods, not even one. Now, you may think, "the small rover is for exploration and to find resources like resin quickly". Yes, but what you thought it is not in the description of the rover: "caution: not suited for research pods".
As a fresh player, one of the first thing that you get in your starting area are enough basic materials around your initial base, basic game design 101, therefore, you don't need to worry too much about them (and basic resources are small). Thus, the second thing that you usually want are research pods to keep going up (or down) the research tree. Nonetheless, you find that carrying research pods are cumbersome and slow. What is the thing that vehicles are good for? Carry stuffs in an easy and fast way. Thus, it makes a beginner to think that a small rover is the ideal candidate to transport research pods.
But there is more. Research pods are kind of rare near your starting base. Thus you want to look for them around fast (again, the ideal job of a vehicle). And tethers are cumbersome to put, retract, and repeat. You want a bit more freedom and the small rover seems to be the ideal candidate for that.
Thus, what a beginner may want from his first rover, he may not get it. An easy transport that is useful and can carry one of the most annoying things in the early game: research pods.
I think that the small rover should be the catapult that pushes the early game into mediums and large rovers. It does not need to be able to carry tons of stuffs. Nor it should be a smaller version of the other two rovers: a train of rovers to gather resources. It is fine as a light explorer, easy on fuel and cheap to build. But it is a trap and its functionality are not suited so much for the first hour or so of gameplay.
Now, there are two solutions for this. The obvious one is to make the small rover able to carry research pods. Only one is just more than enough. You can put as a restriction that it can only carry research pods and smaller things, or that it uses a bit more energy if its carrying a pod or it became a bit slower.
On the other hand, the second solution might be to have smaller research pods. Research pods that are easily carried on your backpack, that gives you enough research points to worth them at the start, but not so much that makes the larger one useless.
Any of these solutions are fine. If there are plans to make more size diverse research pods, go for that, just make them available and a bit more frequent in the starting zone or planet.